I suspect the best option would be something like
this. Alternatively I know in the vintage computer community there are some highly regarded early NEC Multisync monitors that can go down low enough for the likes of CGA - I'm guessing the BBC is similar in its RGB output?
I've got a couple of computers with TTL RGB outputs - Sord M23's and a Apple IIe with what appears to be a relatively rare RGB card. I'm also on the lookout for a BBC, so I suspect at some point I may be tempted with something like the above! Unless there's an easy way to convert RGB to component - but then the only TV here with component in is a 29" Sony CRT so it might be a smidge big!